Eyewitness Amr Saeed said military troops and police forces opened fire on the protesters during early morning prayers.

"We were praying at dawn when the army came with two armoured personnel carriers, two tanks and troops on foot," he said.

"They locked the door and threw bombs and ammunitions. More than 34 were killed and many were wounded," he added.

Injured were taken to a makeshift hospital which was set up in Cairo's Nasr City district. Footage filmed by the Muslim Brotherhood showed corpses lying on the ground, badly injured survivors being treated amid large pools of blood.

A medic from the area said ambulances were unable to transport more than 200 wounded to hospitals because the military had blocked off the roads.

Morsi supporters have been holding rallies and a sit-in outside the Republican Guard building since the military deposed Mr Morsi last week during massive protests against him.